The present invention relates to an automatic transmission for executing speed changes by changing the engagement/disengagement states of a plurality of frictional engagement means such as clutch means and brake means and, more particularly, to an automatic transmission having plural kinds of combinition patterns of the engagement/disengagement of the frictional engagement means for settng one of several speed stages (hereinafter simply referred to as speeds).
An automatic transmission for a vehicle is generally constructed by using a plurality sets of planetary gear mechanisms, by connecting predetermined ones of rotary members of sun gears, ring gears or carriers of the individual sets, by connecting any of the rotary members to an input shaft through clutch means, by holding another rotary member selectively by brake means, and by connecting an output shaft to still another rotary member. In the automatic transmission of this kind, the number of speeds to be set, the gear ratios at the individual speeds, and/or the numbers of revolutions and loaded torques of the rotation members are changed in various manners depending upon the number of planetary gear mechanisms used, the manner of connecting the individual rotary members, or the number of or manner of arrangement of the frictional engagement means such as the clutch means or the brake means so that a remarkably large number of ratios can be made in principle. (The "number of revolutions" is defined herein as the number of revolutions of a rotary member of a gear train for each rotation of an element external to the gear train, e.g., an input member. All conceivable in principle could not be put to practical use. It is not easy to create an automatic transmission which can meet the practical requirements sufficiently such as feasible production, small size and light weight, sufficient speed change controllability or excellent durability. Even an automatic transmission which is enabled to set five forward and one reverse speeds by using three planetary gear sets, for example, may have an overlarge gear ratio at the reverse speed in dependence upon the manner of arranging the frictional engagement means such as the clutches or the gear ratios (i.e., the ratios of the number of teeth of the sun gears to that of the ring gears) of the individual planetary gear sets. It is necessary for this automatic transmission to have high capacity frictional engagement means. This automatic transmission has problems in deterioration of the durability of the frictional engagement means or the bearings. In order to solve this problem, there has been proposed an automatic transmission in which the gear ratio at the reverse speed is made similar to that at the forward first speed, as disclosed in Japanese Patent Laid-Open No. Sho 60-57036.
The structure of the automatic transmission according to this proposal will be briefly described in the following. This structure is constructed mainly of three sets of single pinion type planetary gear mechanisms. The sun gear of the first planetary gear set and the sun gear of the second planetary gear set are always or selectively connected, and the sun gear of the first planetary gear set and the carrier of the second planetary gear set are connected, if necessary, through a clutch. Moreover, the ring gear of the second planetary gear set and the ring gear of the third planetary gear set are connected to each other and to the carrier of the first planetary gear set. Still moreover, the carrier of the second planetary gear set and the sun gear of the third planetary gear set are connected through a clutch. The input shaft is connected on one hand through a clutch to both the sun gear of the first planetary gear and the sun gear of the second planetary gear set and on the other hand through another clutch to the ring gear of the first planetary gear set. On the other hand, the output shaft is connected to the carrier of the third planetary gear set. Moreover, the brake means for stopping rotation is exemplified by both a brake for holding the sun gear of the first planetary gear set and the sun gear of the second planetary gear set, which are connected to each other, and a brake for holding the sun gear of the third planetary gear set. In this automatic transmission disclosed in Japanese Patent Laid-Open No. Sho 60-57036, at the forward first speed, the rotations of the carrier of the second planetary gear set are blocked by engaging the brake for holding the sun gear of the third planetary gear set and simultaneously by engaging the clutch for connecting the carrier of the second planetary gear set to the former sun gear. At the reverse speed, too, the carrier of the second planetary gear set is likewise held. Moreover, the gear ratios at the forward first and reverse speeds have similar values.
Incidentally, in the Specification of Japanese Patent Laid-Open No. Sho 60-57036, the structure, in which a one-way clutch is interposed between the carrier of the second planetary gear set and the casing, is presented in a skeleton diagram. With this presentation, however, there is neither a clutch and brake application chart corresponding thereto nor any description of the speeds to be set. With the structure having the additional one-way clutch, therefore, it is conceivalbe that the individual speeds are set likewise as in other embodiments disclosed therein.
In the automatic transmission of the prior art thus far described, the speeds are composed of five forward and one reverse speeds and of a 2.5th speed and a 3.5th speed so that the automatic transmission can set seven forward speeds and one reverse speed as a whole. However, only a single combination of the engagement/disengagement of the clutches and brakes is available for setting each of the individual speeds. As a result, the automatic transmission of the prior art may have high shifting shocks or may be obliged to execute complex controls so as to reduce the shifting shocks. In other words, the automatic transmission of the prior art is given only one kind of combination pattern of the engagement/disengagement of the frictional engagement means for setting the individual speeds so that it cannot avoid an increase, if necessary at the time of a speed change from a predetermined speed to another, in the rotational fluctuations of any rotary member. As a result, the moment of inertia accompanying the speed change is increased to cause rapid fluctuations of the output torque, thus raising disadvantages that the shifting shocks are high and that the durability of the bearings or the like is reduced. These disadvantages also occur in case of an increase of the number of revolutions of a rotary member having no direct participation in the transmission of the torque when a predetermined speed is to be set. Then, the rotational fluctuations of that rotary member may be increased at the time of the speed change to increase the shifting shocks. Still the worse, these unnecessary rotations will naturally cause friction between the adjuacent members, thus raising another disadvantage that the friction will generate heat and progress wear.
If, on the other hand, even the unnecessary rotations have to cause the fluctuations for the speed change, the automatic transmission has to be equipped with not only the clutch means and the brake means but also an accumulator, a timing value or an electromagnetic valve for controlling the foregoing means. This results in other disadvantages that the speed change controls are complicated and that the system is large-sized to increase the weight and the cost.
Here, in the automatic transmission using a plurality sets of planetary gear mechanisms, clutches are interposed between the rotary members such as the sun gears of the individual planetary gear sets to increase the number of the speeds to be set and the number of the combination patterns of the engagement/disengagement of the frictional engagement means for setting a predetermined speed. As a result, the aforementioned automatic transmission disclosed in Japanese Patent Laid-Open No. Sho 60-57036 is equipped with a clutch interposed between the rotary members of the planetary gear sets, and it may have a plurality of kinds of combination patterns of the engagement/disengagement of the clutches and brakes for setting any speed. If so, however, the abovespecified Laid-Open has failed to describe not only the presence of plural combination patterns but also a method of utilizing the combination patterns of the engagement/disengagement of the clutches and brakes.